Young campaigners were crucial in persuading politicians to change the country's constitution to outlaw marriage between the ages of 15 and 18 with parental consent.
Half of all girls in Malawi are married by the age of 18 - and one in eight by 15. They are more likely to drop out of school, get pregnant early and suffer physical and emotional violence.
That shouldn't still be happening. Child marriage is supposed to be illegal in Malawi after the minimum age for marriage was raised from 15 to 18 in 2015.
But a legal loophole allowed those aged 15 to 18 to continue to marry if they had parental consent.
That loophole has now been closed, with Malawi's parliament voting 131-2 to amend the country's constitution.
"This is a momentous change for future generations of Malawi - and we are so pleased that young people have played a huge part in this success," Lilly Omondi, country director for Plan International Malawi, said in a statement.
She said young campaigners had been key in persuading politicians to act, adding: "For the relevant ministers and other decision-makers to hear from young people themselves was crucial to the process.
"By ensuring that they have had their voices heard, these young people have helped to secure the health and happiness of millions of Malawian girls to come."
Memory Banda, 20, was one of those young people who led the campaign alongside other activists, politicians faith leaders and teachers.
She said: “When my little sister was just 11, she was forced to marry the man who got her pregnant.
"At the time, I was young and thought this was normal. But I quickly realised the devastating impact it had on her when she was further abused in marriage.
"When she came home, I saw the person who had been my little sister wasn’t my little sister anymore.
"Now, together with a team of young campaigners supported by Plan International, we’ve worked with the government to amend the constitution of our country to help end child marriage - once and for all.”
Malawi has the ninth highest rate of child marriage in the world. Africa accounts for 17 of the top 20 countries.